The present invention relates generally to medical laser imaging systems.
Laser imaging systems are commonly used to produce photographic images from digital image data generated by magnetic resonance (MR), computed tomography (CT) or other types of scanners. Systems of this type typically include a continuous tone laser imager for exposing the image on photographic film, a film processor for developing the film, and an image management subsystem for coordinating the operation of the laser imager and the film processor.
The image data is a sequence of digital image values representative of the scanned image. Image processing electronics within the image management subsystem processes the image values to generate a sequence of digital laser drive (i.e., exposure) values, each of which is representative of one of a plurality of intensity levels (e.g., a grey scale) at a discrete pixel location in the image. The image processing electronics scales and maps the range of scanned image values to a range of laser drive values which will produce a useful, continuous tone photographic image. This mapping operation is necessitated by the nonlinear relationship between the input values and their visual representation, and by the nonlinear sensitometric response of the photographic film to different intensities of light. The image management subsystems of laser imagers commercially available from 3M of St. Paul, Minn. include a plurality of stored lookup tables which characterize the relationship between the image values and laser drive values. Each lookup table is configured for one of several types of film and specific image characteristics such as contrast and the maximum and/or minimum densities of the final image. The selected lookup table for a given image is accessed by the image management subsystem as a function of the image values to determine the associated laser drive values.
In addition to selecting a desired lookup table, users of the commercially available 3M laser imagers can adjust the contrast and density levels on images by manually actuating controls interfaced to the image management subsystem. However, these adjustments are made on a trial and error basis with test patterns, an inconvenient and inefficient procedure. Furthermore, the user is only able to exercise a limited degree of control over the overall imaging system transfer function by selecting one of the lookup tables and adjusting the contrast and density ranges implemented by these lookup tables. This approach also fails to account for drifts in the overall system transfer function that can be caused by factors such as the depletion of developer chemicals and lot-to-lot variations between the ideal and actual film sensitometric characteristics.
It is evident that there is a continuing need for improved laser imaging systems. In particular, there is a need for a laser imaging system capable of automatically adapting to variations in media sensitometric characteristics and media development parameters. The system should also be capable of accommodating a greater degree of user control over the overall imaging system transfer function. Any such imaging system must of course be able to accurately and efficiently implement these functions to be commercially viable.
The present invention is a multi-user digital laser imaging system capable of automatically adapting to variations in media sensitometric characteristics, changes in user preferences and changes in the development process. In one embodiment the laser imaging system is configured to image film contained in a film receiving mechanism as a function of digital image values representative of an image, user commands, and film information characteristic of the film. The imager includes an image data input for receiving the digital image values, a user command input for receiving the user commands, and a film information input for receiving the film information. A laser scanner is responsive to digital laser drive values and capable of scanning a laser beam to image the film. Data characteristic of a plurality of transfer functions are stored in transfer function memory. Each transfer function is representative of the relationship between expected imaged film densities and associated image values. Data characteristic of a plurality of film models is stored in film model memory. Each film model is representative of the relationship between expected imaged film densities and associated laser drive values. RAM for storing digital data is also included. A digital processor is coupled to the image data input, user command input, film information input, laser scanner, transfer function memory, film model memory and RAM. The processor accesses the transfer function memory as a function of the user commands to select the data representative of user-desired transfer functions, and accesses the film model memory as a function of the film information to select data representative of the film models for film to be imaged. The processor generates lookup tables as a function of the selected transfer functions and film models, and stores the lookup tables in the RAM. The lookup tables are data characterizing relationships between the laser drive values and the image values. To image the film the processor accesses the laser drive values in the generated lookup tables as a function of the image values, and provides the accessed drive values to the laser scanner. This approach maintains the integrity of the system transfer function with little inconvenience to the user.
In another embodiment of the laser imager, the transfer function memory stores the transfer functions as data characterizing the relationship between a range of cubic roots of expected transmittance values of an image and associated image values. The film model memory stores the film models as data characterizing the relationship between a range of cubic roots of expected transmittance values of an image and associated film exposure values. The digital processor accesses the film model memory as a function of cubic root transmittance values to determine associated film exposure values, and computes laser drive values as a function of the determined film exposure values. The processor generates index tables of data characterizing the relationships between laser drive values and corresponding cubic root transmittance values, and stores the index tables in RAM. The lookup tables are generated by accessing the index tables as a function of desired cubic root transmittance values from selected transfer functions to create lookup tables of data characterizing relationships between the laser drive values and the image input values. Lookup tables can be quickly generated by this system, thereby enabling the convenient use of the system by several users, each of which has different user preferences.
In yet another embodiment, the imaging system includes a film processor for developing the imaged film, and a densitometer for providing information representative of the density of portions of the developed film. Test wedge memory is also included for storing test wedge data characteristic of test wedges. The test wedge data is representative of a range of laser drive values associated with expected film density values characterized by the film models. The digital processor is coupled to the film processor, densitometer and test wedge memory, and periodically executes wedge calibration procedures to correlate the film model being used with the current sensitometric characteristics of the film. During a wedge calibration procedure the processor accesses the test wedge memory and initiates the imaging of test wedges on the film as a function of the laser drive values. The test wedges on the imaged film are developed, and the actual density of the wedges measured by the densitometer. The processor compares the actual densities of the test wedges to the associated expected film density values. The processor then modifies the film model data as a function of the comparison so the film model corresponds to the actual characteristics of the film. Differences between the actual and ideal film sensitometric characteristics, such as those caused by lot-to-lot manufacturing variations and aging, are thereby corrected before the lookup tables are generated. Greater integrity in the overall imaging system transfer function is achieved through the use of this calibration procedure.
In another embodiment of the laser imaging system, the laser scanner includes an attenuator for adjusting the intensity of the scanned laser beam in response to attenuator control signals. Data characteristic of a density patch is also stored in density patch memory. The density patch data is representative of a digital laser drive value associated with an expected predetermined imaged and developed film density. The digital processor is coupled to the attenuator and density patch memory, and executes a density patch calibration procedure during the printing of each image. During density patch calibration procedures the processor accesses the density patch memory and initiates the imaging of a density patch on each film as a function of the laser drive value. After the imaged film is developed, the actual density the patch is measured by the densitometer. The actual density of the patch is compared to the associated expected film density value. The processor then generates attenuator control signals as a function of the patch comparison to minimize the differences between the measured and actual patch densities. Variations in the overall system transfer function, such as those caused by the depletion of the developer chemicals, between wedge calibrations, are thereby compensated.
Another embodiment of the imaging system has a control panel for receiving user-preference commands such as those describing desired image contrasts and maximum density levels. Before generating the lookup tables the processor modifies the selected transfer function on the basis of the user commands. Users therefore have the capability of customizing images to suit their own preferences. Furthermore, since the cubic root transmittance values stored in the transfer function memory are linearly related to the human brightness response, these modifications can be quickly performed by the processor through linear transformations.